1. Field of the Invention
This invention relates to an improved process for desulfurization and denitrification of a waste gas containing sulfur oxides, nitrogen oxides and halogen-containing materials such as hydrogen chloride, hydrogen fluoride, etc., more specifically a waste gas from combustion of coal, containing relatively large amounts of halogens, for example, about several ppm to 200 ppm of halogen components.
2. Description of the Prior Art
Various methods have been developed and examined as to the desulfurization and denitrification technique of various combustion waste gas. Above all, it is expected that the desulfurization and denitrification method by dry process using a carbonaceous adsorbent will be increasingly used because of many advantages in that there is no problem of disposal of waste water and the process can be carried out with a reduced installation area.
The desulfurization and denitrification technique using a carbonaceous adsorbent consists of a method comprising passing a waste gas transversely through a reactor of a moving bed type wherein a carbonaceous adsorbent is vertically moved from the upper part to the lower part, whereby sulfur oxides are adsorbed and removed and nitrogen oxides are decomposed and removed by the aid of ammonia. In this method, when the concentration of sulfur oxides in a waste gas to be processed is low, the desulfurization and denitrification can simultaneously be accomplished in one reactor, but when the concentration of sulfur oxides is high, it is required to use a gas processing apparatus comprising a reactor consisting of a first moving bed and second moving bed, connected in series, in which a carbonaceous adsorbent is moved from the upper part to the lower part, and to effect mainly desulfurization by firstly feeding a waste gas to the first moving bed and passing the waste gas transversely through the carbonaceous adsorbent layer, and then mainly denitrification by adding ammonia to the waste gas passed through the first moving bed, feeding to the second moving bed and passing the waste gas transversely through the carbonaceous adsorbent layer. In this case, the carbonaceous adsorbent is firstly fed to the second moving bed, subjected to denitrification reaction, then fed to the first moving bed to adsorb sulfur oxides and the carbonaceous adsorbent whose activity is lowered by the adsorption of sulfur oxides in the form of sulfuric acid or ammonium salt is discharged from the reactor, followed by subjecting to regeneration and reusing.
In such a desulfurization and denitrification method by dry process, even if halogen-containing materials such as hydrogen chloride are contained in a waste gas to be processed, these materials can be adsorbed and removed together with sulfur oxides when the adsorption capacity of the carbonaceous adsorbent is sufficient. However, these halogen-containing materials are more difficult to be adsorbed on a carbonaceous adsorbent as compared with sulfur oxides and accordingly, when the concentration of sulfur oxides is high and the adsorption capacity of the carbonaceous adsorbent is not sufficient, or when a processing operation is carried out under such a condition that movement of the carbonaceous adsorbent is controlled to adsorb sulfur oxides up to near the saturation adsorption quantity, the halogen-containing materials are hardly adsorbed and are passed through the first moving bed, followed by reacting with ammonia gas added between the first moving bed and the second moving bed to form ammonium halides, entering the second moving bed with the waste gas and adsorbing on the carbonaceous adsorbent in the second moving bed. The carbonaceous adsorbent, on which ammonium halides have been adsorbed, is moved downward in the moving bed and fed to the first moving bed. When the ammonium halide-adsorbing carbonaceous adsorbent is contacted with a waste gas containing a large amount of sulfur oxides in the first moving bed, the sulfur oxides and ammonium halides are reacted to form ammonium sulfate and ammonium sulfite and to again generate hydrogen halide gases. That is, the halogen-containing materials contained in the waste gas are cycled and gradually accumulated between the first moving bed and the second moving bed. When the concentration of halogens is increased in the gas fed from the first moving bed to the second moving bed, the amount of ammonium halides formed by addition of ammonia gas is increased and the ammonium halides are adsorbed in large amounts on the carbonaceous adsorbent in the second moving bed, in particular, on the carbonaceous adsorbent at the gas inlet side, causing problems of increase of the pressure loss and clogging of an inlet louver.